Re: perf tools: Options being passed to clang when building the BPF part of skels

[Arnaldo Carvalho de Melo <acme@kernel.org>]

Em Tue, Aug 22, 2023 at 08:58:09PM +0200, Guilherme Amadio escreveu:
> On Tue, Aug 22, 2023 at 09:33:16AM -0300, Arnaldo Carvalho de Melo wrote:
> > 	On gentoo I'm noticing this:

> > clang-16clang-16: : clang-16error: : error: clang-16clang-16: clang-16clang-16clang-16: clang-16error: clang-16: : ignoring '-fstack-protector-strong' option as it is not currently supported for target 'bpf' [-Werror,-Woption-ignored]: : : clang-16error: ignoring '-fstack-protector-strong' option as it is not currently supported for target 'bpf' [-Werror,-Woption-ignored]error: error: : 
> > error: 
> > error: error: error: ignoring '-fstack-protector-strong' option as it is not currently supported for target 'bpf' [-Werror,-Woption-ignored]ignoring '-fstack-protector-strong' option as it is not currently supported for target 'bpf' [-Werror,-Woption-ignored]error: ignoring '-fstack-protector-strong' option as it is not currently supported for target 'bpf' [-Werror,-Woption-ignored]ignoring '-fstack-protector-strong' option as it is not currently supported for target 'bpf' [-Werror,-Woption-ignored]
> > 
> > ignoring '-fstack-protector-strong' option as it is not currently supported for target 'bpf' [-Werror,-Woption-ignored]ignoring '-fstack-protector-strong' option as it is not currently supported for target 'bpf' [-Werror,-Woption-ignored]ignoring '-fstack-protector-strong' option as it is not currently supported for target 'bpf' [-Werror,-Woption-ignored]ignoring '-fstack-protector-strong' option as it is not currently supported for target 'bpf' [-Werror,-Woption-ignored]

> > ignoring '-fstack-protector-strong' option as it is not currently supported for target 'bpf' [-Werror,-Woption-ignored]

> > And while just doing:
> > 
> > diff --git a/tools/perf/Makefile.perf b/tools/perf/Makefile.perf
> > index a5dd1ba..7726b57 100644
> > --- a/tools/perf/Makefile.perf
> > +++ b/tools/perf/Makefile.perf
> > @@ -1101,7 +1101,7 @@ else
> >  endif

> >  $(SKEL_TMP_OUT)/%.bpf.o: util/bpf_skel/%.bpf.c $(LIBBPF) $(SKEL_OUT)/vmlinux.h | $(SKEL_TMP_OUT)
> > -	$(QUIET_CLANG)$(CLANG) -g -O2 --target=bpf -Wall -Werror $(BPF_INCLUDE) $(TOOLS_UAPI_INCLUDE) \
> > +	$(QUIET_CLANG)$(CLANG) -g -O2 --target=bpf -Wall -Werror -Wno-option-ignored $(BPF_INCLUDE) $(TOOLS_UAPI_INCLUDE) \
> >  	  -c $(filter util/bpf_skel/%.bpf.c,$^) -o $@

> >  $(SKEL_OUT)/%.skel.h: $(SKEL_TMP_OUT)/%.bpf.o | $(BPFTOOL)

> > "Fixes" it, I'm curious as where is that we're setting up the options
> > that are being passed to clang at:

> > tools/perf/Makefile.perf

> > $(SKEL_TMP_OUT)/%.bpf.o: util/bpf_skel/%.bpf.c $(LIBBPF) $(SKEL_OUT)/vmlinux.h | $(SKEL_TMP_OUT)
> >         $(QUIET_CLANG)$(CLANG) -g -O2 --target=bpf -Wall -Werror $(BPF_INCLUDE) $(TOOLS_UAPI_INCLUDE) \
> >           -c $(filter util/bpf_skel/%.bpf.c,$^) -o $@

> > Perhaps it is plain just using CFLAGS? I.e. maybe the native build
> > CFLAGS is being passed to the BPF target clang calls?

> > Ideas?

> In the patches I sent previously, I had a commit to address this. In

I tried the one that introduced CLANG_OPTIONS and checked WERROR= to set
Werror, read more further down this message.

I think we need to have it but for this problem I want to have finer
grained control on what goes into the CFLAGS for CLANG _when building
BPF bytecode_.

> summary, the extra flags you see are hardening flags added by default.
> Have also a look at /usr/include/gentoo/fortify.h. I suppose other 

> distributions may add such flags as well. It's a pity the hardening
> flags are added indiscriminately to all target architectures, but just

Oh, I see, so we need to overcome that in the perf Makefiles by
_removing_ those unsupported options for the BPF target, something like
what is done in tools/perf/util/setup.py, because some of the options
may not be supported for one version of clang but be supported in a
newer one :-\

> making the compiler not error out should be enough to work around the
> issue. In my patch, I put -Werror under the control of the WERROR build
> option and disabled it when configuring perf in the ebuild. Since other

Right, so it is the one I saw.

> warnings may be triggered in the future, having an easy way to disable
> -Werror when building the package would be nice to have, otherwise when

Right, I'll apply that patch, I tested it, it just didn't some the
problem at hand, i.e. after applying it one has to use WERROR=0.

- Arnaldo

> a new compiler comes out users may hit build failures, since we are a
> source based distribution.
> 
> Best regards,
> -Guilherme